I would like to know a good way to change gold for gems. I have tried to buy silver keys and gold keys but, no one sell me the keys for the max price (in gold)

They do. You just got put at the back of the first-in-first-out queue that's several months long because so many people request keys for gold.

I am a level 37, and i would like that other objects can be good for the trade gold/gems.

I need a lot of gems for update the cauldron.

Make precrafts that sell for gems at the tradehouse, such as great Nordic Lutes etc.If you don't want to do that,but want to flip, then you'll run into things such as magic chests, which right now are priced at 27.3M to get 8 gems post-tax (11 pre-tax), so about 3.4M per gem.Other thing: if you're not playing on mobile, consider installing Shop Heroes there and linking your account there; 50+ gems/day from the gem videos should be doable for most. (I get batches of about 5 videos per time I login, so with 5 logins per day, that's 5*5*2=50 gems)